We also examined two other cases to see whether the vertical pattern of basic variables is robust from case to case. Figure 13 first shows the hourly-mean visibility and hourly-mean precipitation in all four regions from 1−10 March 2015, and then gives the GPH−temperature anomalies as well as wind anomalies and specific humidity anomalies. There were three periods with rain exceeding 10 mm h−1 and lasting for longer than six hours, as shown in Fig. 13b, but two periods with low visibility, as shown in Fig. 13a. On 3 March 2015 and late on 8 March 2015, there was no rain but visibility was also relatively low. Rain-fog weather was observed in the two periods. Two short periods indicated by the dotted box were covered by rain and fog, with visibility lower than 1 km for at least an hour in region 4. As shown in Fig. 13c, two warmer and low-pressure periods were separated by three cool and high-pressure periods in the lower boundary over the port. From Fig. 13d, two wet periods (blue shading) were separated by three dry periods (yellow shading), which also experienced changing wind anomalies from southwesterly to northeasterly in the lower boundary over the port. This implies that the warm-low anomalies and the southwesterly wind anomalies and the humidity anomalies can be used to locate and predict the formation of rain and fog. The two short periods also occurred during the change from southwesterly to northeasterly wind anomalies (Fig. 13d). On the other hand, rain with stratus clouds was recorded on 8 March 2015, so the diurnal cycle of climatological visibility was lower in the local afternoon, but the coastal fog on 3 March 2015 can be identified due to no rain and few stratus clouds. The distributions of the two warmer- and lower-pressure periods can also be detected from the EPS products for lead times of several days.
There were two periods with visibility less than 1 km late on 27 and 29 May 2015 (Fig. 14a). The rain in Fig. 14b occurred before the low visibility on 27 May, but basically no rain was observed on 29 May during the low visibility. The two periods can also be indicated by the two warm-low peaks of GPH−temperature anomalies during 26−30 May in the lower troposphere (Fig. 14c). The two wet centers following the change from southwesterly to northeasterly wind anomalies were consistent with the two fog periods (Fig. 14d). In addition, the two low visibility periods occurred with the change from southwesterly to northeasterly wind anomalies. Similarly, the EPS products can be used to indicate these anomalies for lead times of several days.
Figure 14. As in Fig. 13 but for 21–31 May 2015.
As done for the case from 27−28 January 2016, as well as the cases depicted in Fig. 13 and Fig. 14, we can also examine other cases. One example had two periods of lower visibility, with the first weak one from 1−3 June and the second strong one from 7−10 June 2015. The two inversion periods of temperature anomalies and the two wet periods of humidity anomalies in the lower troposphere were observed to closely accompany the two lower visibility periods. The fog period ended at the moment when the southwesterly wind anomaly changed to a northeasterly one. Another case shows that a two-day period of visibility less than 1 km from 4−5 January 2016 was situated within the inversion and the negative GPH anomalies as well as the wettest humidity anomaly and the change from southwesterly to northeasterly wind anomalies. All cases show that low-visibility episodes are the result of comprehensive effects from GPH−temperature anomalies and wind−humidity anomalies. For a longer episode of low visibility of more than several days, it is influenced by stratus clouds and rain, which is generally classified as “advection fog”. So, this period could be rain-fog type weather. On the other hand, a diurnal cycle of visibility could be observed when there are no rain and stratus clouds, which is the so-called “radiation fog”.
To confirm the relationship by using anomalous variables to indicate low visibility at the port, we performed two calculations based on the observed data and reanalysis data. The first involved calculating their correlation coefficients and the other the threat score. Six-hourly series of visibility and anomalous variables from 1 January to 30 June during 2015−16 (total: 363 days, 1452 samples) were used. The correlation coefficients of visibility against 850−925-hPa-averaged temperature, GPH, specific humidity, westerly wind and southerly wind anomalies were calculated. The correlation coefficients of the first four anomalies averaged over 850−925 hPa were −0.31, 0.40, −0.48 and −0.17, respectively, reaching the 99.9% confidence level, but the southerly correlation was −0.03 with a 75% confidence level. This result shows that low-layer temperature, GPH, and humidity anomalies are better than wind anomalies at indicating whether a low-visibility period is possible.
As done by Qian et al. (2016a), we used the TS to quantitatively measure the applicability of these anomalous features for all 1452 samples in the first halves of the two years. The TSs of anomalies averaged over 850−925 hPa were calculated. Different thresholds of anomalies were tested and the threshold that gave the highest TS is given in Table 1. The first test took the best threshold of humidity anomaly larger than 5.4 g kg−1, and the TS was 0.098 with 24 hitting samples, 15 missing samples, and 235 false alarm samples. The TS of a single variable was lower than 0.1. The TS increased to 0.104 when considering both humidity and GPH anomalies, and reached 0.125 when considering both humidity and temperature anomalies. The TS from the three variables of humidity, GPH and temperature anomalies combined was 0.140. These TS results imply that fog occurrence depends on multiple anomalous variables, with different threshold values. The TS is changed when taking these variables at different grids vertically and horizontally. It would be meaningful in the future to see how the TS changes with different lead times, vertical levels and thresholds of anomalies. The results of this TS analysis are a good reference for studies and operations involving NWP forecast data.
Test Best threshold TS Hit Miss False alarm Exp_q' q' ≥ 5.4 g kg−1 0.098 24 15 235 Exp_z' z' ≤ −190 gpm 0.068 20 19 254 Exp_T ' T ' ≥ 6.2 K 0.092 17 22 146 Exp_q'z' q' ≥ 3.16 g kg−1, z' ≤ −18 gpm 0.104 16 23 115 Exp_q'T ' q' ≥ 2.28 g kg−1, T ' ≥ 7.88 K 0.125 12 27 57 Exp_q'z'T ' q' ≥ 2.24 g kg−1, z' ≤ −13 gpm, T ' ≥ 6.24 K 0.140 13 26 54
Table 1. Threat scores (TS) of identifying dense fog (visibility < 1000 m) by using a single variable of humidity anomaly (q'), GPH anomaly (z'), or temperature anomaly (T '), as well as combinations of anomalous variables: (q'z'), (q'T ') and (q'z'T ').
|Test||Best threshold||TS||Hit||Miss||False alarm|
|Exp_q'||q' ≥ 5.4 g kg−1||0.098||24||15||235|
|Exp_z'||z' ≤ −190 gpm||0.068||20||19||254|
|Exp_T '||T ' ≥ 6.2 K||0.092||17||22||146|
|Exp_q'z'||q' ≥ 3.16 g kg−1, z' ≤ −18 gpm||0.104||16||23||115|
|Exp_q'T '||q' ≥ 2.28 g kg−1, T ' ≥ 7.88 K||0.125||12||27||57|
|Exp_q'z'T '||q' ≥ 2.24 g kg−1, z' ≤ −13 gpm, T ' ≥ 6.24 K||0.140||13||26||54|